A nurse told me that drinking iced tea blocks my iron absorption. how is this true if it is?

Sorta: That's sort of true. However, it's not of any consequence if you drink 1-2 cups of black or green tea. It matters more if you drink certain types of herbal tea, lots of tea, or don't have a balanced diet otherwise. If none of that is true in your case, don't worry about it. Standard iced tea (at usual quantities) will not affect your iron absorption to any significant degree.